In a manufacturing process of semiconductor devices or flat panel displays (FPDs), a plasma is used to perform a processing, such as etching, deposition, oxidation, sputtering or the like, so as to obtain a good reaction of a processing gas at a relatively low temperature. Conventionally, a capacitively coupled type plasma apparatus has been widely employed as a single-wafer plasma processing apparatus, especially, as a single-wafer plasma etching apparatus.
Generally, in the capacitively coupled plasma processing apparatus, an upper electrode and a lower electrode are disposed to face each other in parallel in a vacuum processing chamber, a substrate to be processed (a semiconductor wafer, a glass substrate or the like) is mounted on the upper electrode, and a radio frequency voltage is applied to either one of the upper and the lower electrode. Electrons are accelerated by an electric field formed by the radio frequency voltage to collide with a processing gas. As a result of ionization by the collision between the electrons and the processing gas, a plasma is generated, and a desired microprocessing (for example, etching) is performed on the surface of the substrate by radicals or ions in the plasma. At this time, the electrode to which the radio frequency voltage is applied is connected with a radio frequency power supply via a blocking capacitor in a matching unit and thus serves as a cathode. A cathode coupling method in which the radio frequency voltage is applied to the lower electrode, serving as the cathode, for supporting the substrate enables an anisotropic etching by substantially vertically attracting ions in the plasma to the substrate with a self-bias voltage generated in the lower electrode.
In the capacitively coupled plasma processing apparatus of the dual frequency application type, a first radio frequency power of a relatively radio frequency (generally, 27 MHz or greater) for plasma generation and a second radio frequency power of a relatively low frequency (generally, 13.56 MHz or less) for ion attraction are applied to the lower electrode (see, e.g., Japanese Patent Laid-open Publication No. 2000-156370 and U.S. Pat. No. 6,642,149).
The dual frequency application is advantageous in that plasma density and anisotropic etching selectivity can be individually optimized by the first and the second radio frequency power, and also in that the second radio frequency power of a relatively low frequency can effectively prevent or suppress a deposit adhesion during a process in which deposits such as polymer and the like are adhered to an upper electrode. Specifically, when the ions are incident on the upper electrode serving as an anode, a deposited film (and an oxide film, if it exists) adhered to the electrode is sputtered by ion impact. The number of ions used for the sputtering is determined by the first radio frequency. Further, an electric field that accelerates the ions is generated by the second radio frequency power of the relatively low frequency.
In the conventional capacitively coupled plasma processing apparatus of the dual frequency application type as described above, the upper electrode serving as the anode to which no radio frequency is applied is DC-grounded generally. Typically, a processing chamber, which is frame grounded, is formed of metal, e.g., aluminum, a stainless steel or the like, so that the upper electrode can be held at ground potential via the processing chamber. Accordingly, the upper electrode is directly attached to a ceiling of the processing chamber to be integrally assembled thereto or the ceiling of the processing chamber itself is used as the upper electrode.
With a recent trend of miniaturization of design rules for the manufacturing process, a high-density plasma is required to be available at a low pressure for a plasma processing. In the capacitively coupled plasma processing apparatus in which dual frequency powers are applied to the lower electrode, the frequency of the first radio frequency power, which mainly contributes to a plasma generation, tends to be gradually increased and a frequency of 40 MHz or greater is standardly used in recent years. However, if the frequency of the radio frequency power becomes high, a radio frequency current is made to be concentrated on a central portion of the electrode, so that a density of a plasma generated in a processing space between two electrodes becomes higher at the central portion of the electrode than that at the edge portion thereof. As a result, there occurs a problem that process characteristics become nonuniform in a radial direction. Meanwhile, since the frequency of the second frequency power that mainly contributes to ion attraction is relatively low, it is not focused on the central portion of the electrode. In other words, in the conventional apparatus in which the upper electrode is directly attached to or formed integral with the processing chamber to be DC-grounded therethrough, the functions of the second radio frequency power that include attracting the ions toward the substrate and suppressing the deposit adhesion to the upper electrode are not deteriorated.